November 26, 2008

One of the treats of my childhood was the arrival each week of LIFE magazine, with its amazing photographs and engaging articles. I learned about dinosaurs, galaxies, Hollywood stars and epic movies, sports events, episodes in U.S. history, and many other topics while leafing through the issues. The large photos and detailed illustrations were often strikingly memorable.

Now I can experience those fascinating photos again. Google is digitizing the entire photographic archives of LIFE magazine—10 million photos in all, most of which were not published and have not been seen previously by the public. The images—negatives, slides, glass plates, etchings, and prints—have been sitting in dusty archives for years. About 20 percent of the collection is already online (http://images.google.com/hosted/life).

The images are organized by decade (when the photos were taken) and by category (people, places, events, sports, and culture). When I searched the photo archive for "mathematics," the results yielded dozens of intriguing and beguiling images.

November 11, 2008

A ladder leans against a wall. It begins to slide, the top end moving down the wall and the bottom end across the floor away from the wall.

Such a scenario is the basis for a variety of mathematics problems. For example, if the ladder's bottom end moves away from the wall at a constant speed, what is the velocity of the top of the ladder at any given instant? Curiously, the mathematical model indicates that the velocity of the top approaches infinity when the ladder hits the floor.

How can that be? Even with a smooth wall and a frictionless floor, conservation of energy alone dictates that the velocity can't approach infinity. The answer lies not in the mathematics, which is correct, but in the physics of the ladder's motion.

It turns out that the ladder loses contact with the wall when the top has slid one-third of the way down. The original equations describing the situation no longer apply. The ladder then continues to move away from the wall, in a motion determined only by gravity and the reaction force of the floor. A different set of equations describes this part of the motion.

"Calculus teachers are familiar with the sliding ladder problem from the study of related rates," Kapranidis and Koo write. "However, it is not well known that some versions of this problem are physically impossible."

What happens when the ladder is somehow linked to the wall and can't leave it as it slides down? In this case, the velocity of the bottom increases to a maximum (when the ladder is one-third of the way down the wall), then decreases to a zero by the time the ladder hits the floor. The velocity of the top increases from zero to some finite value.

What conditions do you need for the top of the ladder to maintain contact with the wall and the bottom to move at a constant speed. Kapranidis and Koo note that such a motion can be realized by introducing extra forces to pull down on the top and push back at the bottom.

"This situation does yield arbitrarily large speeds," the authors say. "Achieving this motion requires the application of constraint forces that approach infinity." In other words, this motion can't happen.

"It is only when we consider the forces that cause the motion that we can resolve the paradox of speeds approaching infinity," Kapranidis and Koo conclude. "They also indicate caution in regarding the sliding ladder as an example of a real world application of mathematics."

References:

Freeman, M., and P. Palffy-Muhoray. 1985. On mathematical and physical ladders. American Journal of Physics 53:276-277.

About

Welcome to an occasional series devoted to "cool stuff" that I encounter while browsing the world of mathematics and computer science. I'll peek at new developments in math and its applications, and I'll revisit old puzzles, famous problems, and historic events—anything mathematical that happens to catch my eye. I hope you'll find something of value in these brief, informal forays into the world of math.

About Me

Ivars Peterson is a freelance writer and editor. He was Director of Publications at the Mathematical Association of America from 2007 to 2014. As an award-winning mathematics writer, he previously worked at Science News for more than 25 years and served as editor of Science News Online and Science News for Kids. His books include The Mathematical Tourist, Islands of Truth, Newton's Clock, and Fragments of Infinity: A Kaleidoscope of Math and Art.